This invention relates generally to air data systems that are incorporated into air vehicles, and more specifically, to methods and systems for using ratiometric characterizations to improve accuracy of air data systems.
Air data parameters are important in maintaining a high quality of performance of air vehicles. Three air data parameters include altitude, velocity, and mach number.
Pressure altitude is a function of static pressure and is used to determine a quantity of flight conditions. For example, a decrease in pressure is typically indicative of an increase in altitude. Also, as pressure decreases, air density decreases. Air density is also a function of static air temperature. For example, if one plane is flying at sea level and one is flying at 10000 feet they both could be indicating 300 knots but the higher vehicle is actually flying faster. This phenomenon becomes more pronounced at higher altitudes.
Velocity is a function of impact pressure (i.e. the total pressure minus the static pressure). Velocity is the most common parameter used to control air traffic and other aircraft maneuvers. Velocity, as well as air density, is used to control fuel consumption and required power needed to fly at cruise conditions.
The mach number is the ratio of air vehicle speed and the speed of sound. As the air vehicle moves through the air, the air molecules near the air vehicle are disturbed and move around the air vehicle. If the air vehicle is moving at a relatively low speed, for example, less than 250 mph, the density of the air flow remains relatively constant.
At higher air vehicle speeds, some of the energy from the air vehicle compresses the air and locally changes a density of the air. This compressibility effect alters the amount of the resulting force on the air and becomes more important as speed increases. Near and beyond the speed of sound, about 330 meters per second or 760 mph at sea level, small disturbances in the flow are transmitted to other locations. Such disturbances have a constant entropy. For example, a sharp disturbance may generate a shock wave that could affect both the lift and drag of the air vehicle. As a result, the mach number is an important air data parameter that is used to control the performance of the air vehicle. The mach number also changes as a function of altitude.
Some air data systems, for example, those utilized in high performance aircraft are highly accurate, and thus relatively expensive. However, for lower cost air vehicles such as missiles, drones, and unmanned aerial vehicles (UAVs), accuracy of the air data system, while important, may be lessened in order to meet cost constraints that may be associated with such air vehicles. However, it is difficult to even meet moderate accuracy requirements for air data systems that incorporate low cost commercial sensors because of the sensitivity of such sensors to temperature and pressure.